1. Field of the Invention
The present invention relates to a wavelength division multiplexing optical communications technology, and more particularly to a technique for making transmission characteristics uniform for transmitting all optical signals with the same transmission characteristics.
2. Description of the Related Art
In a wavelength division multiplexing (WDM) optical communications system, transmitting all optical signals with the same transmission characteristics is referred to as optimization of transmission characteristics. In the wavelength division multiplexing optical communications system, as shown in FIG. 1, the deterioration of transmission characteristics occurs in a transmitter, a transmission line, and a receiver. Moreover, the deterioration conditions of transmission characteristics differ between optical signals.
Furthermore, when operating a system, the deterioration conditions of transmission characteristics are different between optical signals, due to various causes, such as the repair of an amplifier or a cable, which results from repairs conducted in a transmission section, and the deterioration of a fiber due to age.
Therefore, pre-emphasis must always be set for each optical signal at the transmitting end, and transmission characteristics must always be optimized at the receiving end. Note that pre-emphasis refers to controlling the power of each optical signal.
The difference in transmission characteristics between optical signals can be observed at the receiving end, as differences in an OSNR (Optical Signal to Noise Ratio), BER (Bit Error Rate), or Q-value.
FIG. 2 shows in the prior art the relation between pre-emphasis and an OSNR after transmission.
Making an OSNR uniform at the receiving end so as to optimize transmission characteristics is a well-known technique in the prior art. In this technique, the OSNR difference between optical signals, which is calculated by monitoring the OSNR for each optical signal at the receiving end, is fed back to the transmitting end as a pre-emphasis value, thereby enabling pre-emphasis to be set at the transmitting end. Adjusting an OSNR by directly changing the output power of each optical signal is a general method for setting pre-emphasis.
As a result, if a transmit light spectrum 1, for which no pre-emphasis is set, is transmitted on a transmission line, the OSNR of a receive light spectrum 1, which corresponds to the transmit light spectrum 1, varies greatly. However, if a transmit light spectrum 2, for which pre-emphasis is set, is transmitted on a transmission line, the variance of the OSNR of a receive light spectrum 2, which corresponds to the transmit light spectrum 2, is reduced.
FIG. 3 shows the configuration of a transmitter of the prior art. FIG. 4 shows the configuration of a receiver of the prior art.
First, operations of the transmitter, which has a configuration as shown in FIG. 3, are explained below.
The transmitter has, for each wavelength, a laser diode driver (LD DRIVER) 1201, a laser diode (LD) 1202, an attenuator (ATT) 1203, a coupler (CPL) 1204, a post amplifier (POST AMP) 1205, and a photodiode (PD) 1208. The laser diode driver 1201 drives the laser diode 1202, while adjusting the output power and wavelength corresponding to each optical signal. The optical signal outputted from the laser diode 1202 is inputted to the post amplifier 1205 via the attenuator 1203 and the coupler 1204, and the optical signal is amplified in the post amplifier 1205. The optical signals, each of which is outputted from the post amplifier 1205, are multiplexed by an arrayed waveguide grating (AWG) 1206, and the multiplexed optical signal is outputted to a transmission line via a coupler 1207.
In the configuration shown in FIG. 3, the coupler 1204 branches part of each optical signal to the photodiode 1208, resulting in part of the optical signal being detected by the photodiode 1208. The detection results are inputted to a CPU 1210. Meanwhile, part of the transmitted optical signal, which is outputted from the AWG 1206 to a transmission line, is branched and inputted to an optical spectrum analyzer 1209 by the coupler 1207. The optical spectrum analyzer 1209 monitors the peak power and wavelength of the transmitted optical signal, and notifies the CPU 1210 of the results. The CPU 1210 controls the laser diode driver 1201 and attenuator 1203 based on the output from the photodiode 1207 and optical spectrum analyzer 1209, for each optical signal.
Next, operations of the receiver, which has a configuration as shown in FIG. 4, are explained below.
At the receiver, an optical signal received through a transmission line is inputted via a coupler 1301 to an AWG 1302, where the optical signal is demultiplexed into optical signals of various wavelengths.
The receiver has, for each wavelength, a filter 1303 for separating an optical signal of a specific wavelength, an inline amplifier (INLINE AMP) 1304, a dispersion compensating fiber (DCF) 1305, an optical-electrical converter (O/E) 1306, a forward error corrector (FEC) 1307, and an electric-signal demultiplexer (DEMUX) 1308.
In the configuration according to the prior art, as shown in FIG. 4, the coupler 1301 branches part of a received optical signal into an optical spectrum analyzer 1309. The optical spectrum analyzer 1309 measures the OSNR for each optical signal received, and notifies a CPU 1310 of the results. The CPU 1310 feeds back the OSNR differences between optical signals received, as a pre-emphasis value, to the transmitting end by using a prescribed communications line.
However, in the transmitter, which has a configuration as shown in FIG. 3, the CPU 1210 receives the above-mentioned pre-emphasis value, and controls the laser diode driver 1201 for each optical signal, based on the pre-emphasis value.
As stated above, the prior art is aware of an OSNR so as to optimize transmission characteristics, and makes uniform only an OSNR used for all optical signals. Usually, the most important factor of the transmission characteristics in digital transmission is a transmission error rate. Therefore, it is important to make uniform a transmission error rate for all optical signals in the optimization of transmission characteristics. However, in the prior art, even if an OSNR is made uniform for all optical signals, the transmission error rate is not necessarily made uniform for all optical signals.
Thus, with respect to a transmission error rate, the examples of which are a BER and a Q-value, the prior art has a problem, as shown in FIG. 5A, in that even if the OSNR is made uniform for optical signals 1, 2 and 3, the transmission error rate does not become uniform because of the difference of Q-values of the optical signals.
Furthermore, in the prior art, to set pre-emphasis at the transmitting end, the CPU 1210 directly changes the output power of the laser diode 1202 by controlling the laser diode driver 1201, for each optical signal. However, this method has a problem in that the setting of pre-emphasis for each optical signal must be repeated, while maintaining the power balance of optical signals, because the peak power of the other optical signals simultaneously change, resulting in the set value of pre-emphasis for each optical signal deviating from a proper value.
In view of the above background, the present invention aims at achieving real optimization of transmission characteristics, by making uniform a transmission error rate for all optical signals at the receiving end, based on the adjustment of an OSNR at the transmitting end.
The present invention supposes an apparatus or method for making uniform transmission characteristics in the wavelength division multiplexing optical communications system.
The apparatus according to a first aspect of the present invention has the following configuration.
First, the relation between changes in a signal-to-noise ratio and changes in a transmission error rate at the receiving end is calculated for each optical signal to be wavelength-division-multiplexed.
Next, based on the relation, the signal-to-noise ratio for each optical signal is changed so as to attain a uniform transmission error rate for all optical signals at the receiving end.
The apparatus according to a second aspect of the present invention has the following configuration.
First, for each optical signal to be wavelength-division-multiplexed, the difference between a reference value, which is the value of a signal-to-noise ratio corresponding to the target lower limit of a transmission error rate at the receiving end, and the value of a current signal-to-noise ratio is calculated as a margin.
Next, for each optical signal, a signal-to-noise ratio is controlled so that the margin of the signal-to-noise ratio becomes equal to a prescribed value which is obtained from the margins calculated for the optical signals.
The apparatus according to a third aspect of the present invention has the following configuration.
First, the initial value of a signal-to-noise ratio is stored for each optical signal to be wavelength-division-multiplexed.
Next, for each optical signal, an amplified spontaneous emission noise is superposed on the optical signal so as to gradually reduce the signal-to-noise ratio until a transmission error rate at the go receiving end decreases to a target lower limit.
When the transmission error rate at the receiving end decreases to the target lower limit, the value of the corresponding signal-to-noise ratio is stored as a target lower limit, for each optical signal.
Subsequently, for each optical signal, the difference between the stored initial value and the stored target lower limit is calculated as a margin.
Next, for each optical signal, the difference between a prescribed value which is obtained from the margins calculated for the optical signals, and the stored initial value is calculated as the pre-emphasis amount of the signal-to-noise ratio corresponding to the optical signal.
Then, for each optical signal, the signal-to-noise ratio is controlled by superposing an amplified spontaneous emission noise corresponding to the pre-emphasis amount on the optical signal.
Here, the above-mentioned superposition of the amplified spontaneous emission noise on the optical signal is conducted at the transmitting end. Alternatively, it is possible to provide the apparatus with a configuration in which the above-mentioned superposition is conducted at the receiving end.
In the above-mentioned configuration of the invention, the prescribed value, which is obtained from the margins for optical signals, can be the average of the margins for the optical signals.
In the above-mentioned configuration of the invention, either a Q-value or a bit error rate can be used as a transmission error rate.
According to the above-mentioned configurations of the invention, it is possible to achieve real optimization of transmission characteristics, because the relation between changes in a signal-to-noise ratio and changes in a transmission error rate at the receiving end is calculated for each optical signal so as to optimize transmission characteristics, and based on the calculation results, the signal-to-noise ratio for each optical signal is changed so as to make uniform a transmission error rate for all optical signals at the receiving end.